A Class-D audio amplifier is more efficient than a linear amplifier, such as a Class A/B amplifier, since its output stage operates in switch mode, thereby minimizing power consumption. The rectangular waveform produced by the output stage can be used to power an audio speaker. Sometimes, additional filters are added between the output stage and the audio speaker. The filtered signal is an amplified version of the input audio signal.
It has become more common that audio signals are in digital format and deciphered by a system decoder. The digital audio signals are processed by an audio processing system containing a class D audio amplifier to generate high power audio signals to drive a speaker.
In a conventional method, illustrated in FIG. 1A, the audio processing system contains a high resolution/high speed D/A converter and a closed-loop Class D audio amplifier. The high resolution/high speed D/A is required to convert an audio signal from the digital format into the analog format. The class D audio amplifier then amplifies the analog audio signal to drive a speaker. An audio output with very low distortion can be obtained. The disadvantage of this method is that the analog audio input of the audio amplifier is subject to noise which in turn degrades the quality of the output signal.
In another conventional method, illustrated in FIG. 1B, the audio process system contains a complex pulse width modulation (PWM) modulator, a high resolution/high speed A/D converter, and an open-loop power stage. First, the PWM modulator converts an audio signal from the digital format to the PWM format. In order to improve the power-supply rejection ratio (PSRR) and overcome the non-linear distortion in the output switching network, the output signal must be fed back through a high speed and high resolution A/D converter to the PWM modulator. This method avoids the noise associated with the analog audio input signal. However, the complex PWM modulator and the high speed A/D converter are costly components.
Accordingly, a simpler solution is needed to achieve a high quality output signal in Class D audio amplifier application with minimized noise interference.